The present invention relates to a steam turbine in which at least two turbine sections, in combination, selected from a high pressure turbine section, an intermediate pressure turbine section and a low pressure turbine section is accommodated in a single turbine casing.
There has been attempted to realize a steam turbine having a main steam pressure of 100 kg/cm.sup.2 or more, a main steam temperature of 500.degree. C. or more, and a rated output (power) of 100 MW or more, which are rotated at a rotation speed of 3,000 rpm in the case of being equipped with a last-stage movable blade of turbine having an effective blade length of 36 inches or more, or which are rotated at a rotation speed of 3,600 rpm in the case of being equipped with a last-stage movable blade of turbine having an effective blade length of 33.5 inches or more. In such steam turbine, a set of a high pressure turbine section, an intermediate pressure turbine section and a low pressure turbine section or a set of a high pressure turbine section and a low pressure turbine section is provided on a single turbine rotor (turbine shaft) supported by two journal bearings placed on a pedestal, each of these turbine sections being integrally accommodated in a single turbine casing. However, such steam turbine has not yet been bought into practice at present and remain on the drawing board because of their technical difficulty, particularly, difficulty of preventing shaft vibrations caused by insufficient stiffness of the shafting in association with the increased bearing span.
A steam turbine satisfying the above-mentioned design requirements may have a configuration as shown in FIG. 17, for example.
In this steam turbine, a turbine casing 1 has a double casing structure consisting of an outer casing 1a and an inner casing 1b, and in the inner casing 1b of the double casing structure, for example, a high/intermediate pressure integrated turbine rotor 4 having a high pressure turbine section 2 and an intermediate pressure turbine section 3 is accommodated. On the other hand, a low pressure turbine casing 5 also has a double casing structure consisting of an outer casing 5a and an inner casing 5b, and a low pressure turbine rotor 7 having low pressure turbine sections 6a, 6b, in which steams flow in directions opposing to each other, is accommodated in the inner casing 5b of the double casing structure. The low pressure turbine rotor 7 and the high/intermediate pressure integrated turbine rotor 4 are connected with each other through a coupling 8.
In another steam turbine, for example, as shown in FIG. 18, a high/intermediate pressure integrated turbine rotor 4 is accommodated in the inner casing 5b of the double casing structure such as describe above, while a low pressure turbine rotor 7 having a low pressure turbine section 6, in which a steam flows as a single flow, is accommodated in an inner casing 5b of a low pressure turbine casing 5.
The low pressure turbine casings 5 shown in FIGS. 17 and 18 both are formed with a conical recess portion 11 at the position where the low pressure turbine rotor 7 is inserted in a turbine exhaust hood 10 (chamber or section) defined by a partition wall 9 to ensure an installation area for a journal bearing 12, and the turbine exhaust hood 10 is connected to a condenser (not shown) on its downstream side.
Furthermore, in the steam turbines shown in FIGS. 17 and 18, the high/intermediate pressure integrated turbine rotor 4 and the low pressure turbine rotor 7 are supported by three or four journal bearings 12.
On the other hand, even in the case of a steam turbine which employs, for example, the high/intermediate pressure integrated type turbine which does not satisfy the above-mentioned design requirements, for example, as shown in FIG. 19, a high/intermediate/low pressure integrated turbine rotor 4a having a high pressure turbine section 2, an intermediate pressure turbine section 3 and a low pressure turbine section 6 is supported by journal bearings 12 placed on pedestals 13a, 13b. The turbine exhaust hood 10 defined by a partition wall 9 is formed with a conical recess portion 11 and connected to a condenser, not shown, on its downstream side. In this case, since a bearing span S of the journal bearings 12 supporting the high/intermediate/low pressure integrated turbine rotor 4a is relatively short, it is possible to satisfactorily handle the problem of vibrations that occur during the operation.
Generally, in a steam turbine, as the output power increased because of increase in the pressure and temperature of the steam to be supplied, the number of turbine stages consisting of combination of turbine nozzles and turbine movable blades is increased to thereby respond to the increased power, so that the bearing span S of the turbine rotor is inclined to become long. For this reason, in the case of the high/intermediate/low pressure integrated turbine 4a having e.g., the high pressure turbine section 2, the intermediate pressure turbine section 3 and the low pressure turbine section 6 on a single shaft, the bearing span S becomes long. Accordingly, providing that a shaft diameter of the high/intermediate/low pressure integrated turbine 4a is defined as D.sub.O, as the ratio of the shaft diameter with respect to the bearing span S (S/D.sub.O) becomes higher, the stiffness of the shaft becomes lower, and according to the lowering of the eigenvalue (characteristic value) frequency of the shaft, the critical speed becomes lower, thus, making it difficult to satisfactorily operate the steam turbine.
Particularly, for bringing such a steam turbine into practice that has a main steam pressure of 100 kg/cm.sup.2 or more, a main steam temperature of 500.degree. C. or more, and a rated output of 100 MW or more, rotated at a rotation speed of 3,000 rpm in the case of being equipped with a last-stage movable blade of turbine having an effective blade length of 36 inches or more or rotated at a rotation speed of 3,600 rpm in the case of being equipped with a last-stage movable blade of turbine having an effective blade length of 33.5 inches or more, and that employs a single turbine rotor supported by two journal bearings placed on pedestals, if the conventional technique is directly applied, the bearing span S becomes long to lower the critical speed, and in particular, as the secondary critical speed approaches the rated rotation speed, vibrations of the shaft is increased, which can hinder the operation.